Python 3.10+ compatibility

[JelleZijlstra]

One of my goals in this fork was to enable compatibility with more recent versions of Python (3.10 and up). The problem is that apns2 currently relies on the unmtaintained hyper library, which fails to import on 3.10+.

I looked into two approaches:

• Port apns2 to use httpx (Switch to httpx #2, based on previous work in Switch from hyper to httpx (for Python 3.10 support) Pr0Ger/PyAPNs2#149). However, this PR does not update the test suite to work with the new code, and I'm not comfortable enough with APNS2 to do so myself.
• Resurrect hyper and make it compatible with 3.10+. I started https://github.com/compat-fork/hyper for this, but it's not trivial to even get the tests running on Python 3.8, and it feels irresponsible to release an HTTP client library with possible security vulnerabilities, so I probably won't pursue that further.

For now, this fork will support only Python 3.8 and 3.9. If someone contributes a way to make it work with a more modern HTTP library, either httpx or something else, I'd accept the PR.